Colonel Decker was the best troubleshooter the U.S. army had in two wars, but then extreme political pressures similar to those that resulted in the A-Team's trial resulted in his career being stalled. He was given an opportunity to rehabilitate himself by bringing in the A-Team by any means necessary.
Decker and Hannibal had a bit of a past. They mixed it up once in the Doom Club (the Da Nang officers' open mess) over the fact that Decker blew up Cong hospitals.
Decker: It seems the very methods that made me an embarrassment and shuffled me off to the side are the exact reasons I'm being considered for this job.
Decker: Hurry up! Get after him! The one who catches him keeps his rank!
Face: Who in the world is this guy? Sure isn't Lynch!
Hannibal: I dunno, but it's gonna be fun finding out, huh?
B.A.: The man's as crazy as you are, Hannibal!